Semiconductor devices used in high performance applications are often compound semiconductor devices. Group III-V semiconductor devices, such as GaAs and GaN, are typical compound semiconductor materials used in high performance applications. GaAs devices are often used in high voltage, high power, and high frequency products. Volume production for more widespread application of such products has called for compound semiconductor device products to be small in size and cost competitive.
Interest in fabrication cost reduction has lead to low cost packaging of compound semiconductor devices. High power and high frequency devices were traditionally packaged in hermetically sealed, cavity-type packages. Such packages usually provide adequate moisture protection at the chip level. While cheaper, plastic packaging does not provide a good moisture barrier. Plastic packages may thus leave moisture protection to the chip itself.
Ohmic fences have been used to provide moisture protection in GaAs devices and products with plastic packaging. Unfortunately, circuit layouts with ohmic fences were unable to prevent chip level failures during conventional temperature, humidity, bias (THB) testing. Humidity penetrating through the plastic packaging lead to the failures.
Changes to the circuit layout and fabrication process have also been unsuccessful. For example, a moisture resistant barrier between a polyimide layer and a metallization line may retard corrosion of the metallization line by blocking water released from polyimide layer. Despite such efforts, humidity penetration problems still persist.
Other efforts have focused on configuring bond pads with a protective metallic layer to block moisture. Even if successful, such efforts fail to address moisture penetration from the edge and the top of a chip.